Read Matthew 19: 1-11. The verse deals with divorce but might offer some explanation to your question.

In verse 8 Jesus says, "“Moses permitted divorce only as a concession to your hard hearts, but it was not what God had originally intended.”

God allowed these things because humans were so sinful. Divorce was not what God intended but because the Jews had become to heard hearted towards God he allowed them to have divorces. The same goes with having multiple wives.

If you read the majority of people having more than one wife, it was due to someone not doing what they should have done in the first place. Selfishness, lust, power, etc.

You also have to think about the culture at the time. A lot of men had "multiple wives," but most of them might have had absolutely nothing to do with sex (not saying that didn't exist, but that goes back to my previous point). It more had to do with protecting woman from living a life of shame, prostitution or poverty. In David's case, most of the "wives" he got from Saul probably didn't even live in the same place as him.